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ApplicationNote

Understanding Advanced P25 ControlChannel Functions

This application note deals with an advanced form ofP25 trunking using explicit messaging.1

Explicit messaging is currently used in the VHF and UHFfrequency bands and it is a feature that allows trunkedP25 networks to fully exploit the flexibility that the VHFand UHF bands offer. As a precursor, we want toreview some of the basic terminology regarding P25networks, especially the difference between implicit andexplicit modes of operation.

Implicit operation means that the radio's own internal

programming in conjunction with the P25 network mes-sages will determine what channel/frequency pair theradio assigns to a particular network channel. This isalso known as the "Short Channel form". The explicitmode of operation assigns the actual channel/frequen-cy over the air by providing the exact TX and RX fre-quency assignments directly to the radio.Note: Before reading this application note, it is highly recommended thatthe reader review the companion application note "Understanding 800MHz and VHF/UHF Implicit P25 Control Channel Functions using the2975". This will remove the need to review the basics of trunked radiooperation and provide the reader with an understanding of control channel functions and the P25 implicit mode of operation.

Typically, the implicit mode of operation uses a technique calledsingle block messaging, where explicit typically uses multi-blockmessaging to convey trunking messages. However, certain singleand multi-block messages can be used on either type of network.

Review of Key Terminology

In any wireless mobile communication system, acronyms abound.P25 is no different and there are certain acronyms and termsused that the reader needs to know before using the 2975 fortesting P25 radio systems. The following is a list of the most com-mon acronyms:

ARQ Automatic Retry Request

CAI Common Air Interface

CRC Cyclic Redundancy Check

EIA Electronic Industries Alliance

EXPLICIT The means in which the SU and FNE utilizemessaging to channel assignments usingadvanced control channel messaging techniques

FNE Fixed Network Equipment

GROUP ID A collection of users assigned a particular IDnumber to designate them as a unique category of users

ID Identifier

IMPLICIT The means in which the SU and FNE utilizemessaging to access pre-programmed chan-nel parameters to establish channel assign-ments

INTERCONNECT A call made to or from the Public SwitchedTelephone Network

ISP Inbound Signaling Packet

ISM Inbound System Message - occurs from themobile to the NTS

LB Last Block Flag - indicates if this block is last ormore are to follow

LC Link Control Word

LCF Link Control Format

LCO Link Control Opcode

LDU1 Logical Link Data Unit 1

LRA Location Registration Area

LSB Least Significant Bit

MBT Multiple Block Trunking

MFID ManuFacturer's Identity

MI Message Indicator

MOBILE For this application note, a mobile denoteseither a portable or mobile.

ms Millisecond

MSB Most Significant Bit

NARROW BAND 12.5 kHz bandwidth operation

TX Transmit

Octet 8 bits of information used in messaging

Opcode Identifies the message type

OSP Outbound Signaling Packet

P Protected Flag

PARAMETRICS The RF performance of a radio system includ-ing RF Error, Power and Modulation Accuracy,as well as radio sensitivity

PROTOCOL A series of communication processes definedto set up calls and to send various forms ofdata to and from the mobile.

PSTN Public Switched Telephone Network

REGISTRATION The initial handshake process that allows theradio to communicate

RFSS RF Sub-System

RX Receive

SCCB Secondary Control Channel Broadcast

SS Status Symbols

SU Subscriber Unit - the mobile or portable radio

SYS ID System Identifier

TIA Telecommunications Industry Association

TILE This applies to the 2975. A tile is window viewof various functions within the 2975. There aretwo tiles. One is a 1/4 tile and the other is 1/8tile. The 1/4 tiles are used for displayingrepeater simulators, minimized spectrum analyzers and oscilloscopes, and take up 1/4of the total screen area. 1/8 tiles are used formeters and take up 1/8 of the total screenarea.

TSBK Trunking Signaling BlocK

WACN ID Wide Area Communication Network Identifier


P25 Control Channel Functions

The P25 control channel is a separate channel that is designatedas the control resource and is different from the traffic channel inthat it functions as a resource allocation and digital communica-tion message bearer and handler between the RFSS (RF Sub-System) and the SU (Subscriber Unit).

The P25 control channel feature for trunking is not specific (pro-prietary) to any P25 manufacturer or P25 RFSS configuration. Allmobiles supporting P25 formats should be able to access a P25trunking function as either standard configuration or as an option-al mode of operation, depending on manufacturer and their targetmarkets.

In the P25 standard, the control channel maintains compatibilitybetween conventional operation in that the modulation formatand the bit rate are the same. In the case of P25, the controlchannel is a 9600-baud (data rate) channel using C4FM modula-tion to transfer the digital 1s and 0s to the receiving radio.

Inbound and Outbound Control Channel SignalingMessages

The P25 control channel communicates a variety of informationthrough signaling messages to the SU so that proper access tothe system can be achieved. The P25 standard utilizes systemstatus (overhead messages) and control messages to accomplishthis.2 Correspondingly, the SU sends control messages to the net-work to establish and update communications .

Explicit Versus Implicit Mode of Operation

In a P25 radio system, channel/frequency combinations can beprogrammed into the radio. These programmed values match thenetwork channel/frequency combinations. In the channel grant,the system tells the radio, through the use of a channel identifier,what channel to select. In the implicit mode of operation, theradio internally looks up the channel identifier, and then tunes tothe requested channel.

Implicit messaging typically uses single block TSBK messages.This is a relatively simple means of getting the proper informationto the radio from the control channel. This works fine in bandswith fixed channel assignments like the 800 MHz band (implicitmessaging can be used in undefined bands as well, such as theVHF and UHF bands).

However, in undefined bands, the implicit mode of operation canlimit the use and the inherent flexibility of the band. This is causedby the radio having to be pre-programmed for all the variouschannel/frequency combinations. If the channel is not pre-pro-grammed, the radio cannot tune to that channel because it issimply not aware of its availability.

To remove this limitation, the explicit mode of operation wasdeveloped. This mode allows for direct assignment of radio TXand RX parameters to the radio from the network. Let's look at anexample to see why this is so important.

Let's say that a mobile user is on a VHF or UHF network in Seattle.The radio in use only supports the implicit mode of operation andhas the predefined look-up table of assigned frequencies pre-pro-

grammed so that it can react appropriately when the networksends the channel grant. As long as the user is in the Seattle area,with the assigned frequencies within the VHF or UHF band, theradio can communicate with the network.

However, what happens when the user moves out of the Seattlearea, and needs to access the network? Let's say that the newlocation is Atlanta, and the individual wants to access the Atlantanetwork. What happens to the radio?

Most P25 radios can handle multiple pre-programmed controlchannels. This eliminates any problems in finding a control chan-nel on an established network with an associated WACN ID or SYSID that the radio recognizes. However, the Atlanta network, dueto frequency allocations that are different than the Seattle VHF orUHF band, uses different channel/frequency combinations. Whatdoes the radio do? Inherently, the radio may find the controlchannel. However, it cannot relate the channel grants receivedfrom the network to the appropriate user channels to establishvoice or data communications.

That is where explicit messaging comes into play. With explicitmessaging, the radio can find the appropriate control channelthrough a pre-programmed control channel look up, but whenthe channel grant is received, the network specifically assigns tothe radio, the appropriate TX and RX frequencies that are to beused for voice or data communications.

As the reader can well imagine, depending on the message, theexplicit mode of operation requires more robust messaging fromthe network to accomplish its task. Some messages can use sin-gle block formats to relay the explicit channel information (forexample, the Group Voice Channel Grant Update - ExplicitChannel Form). Others need more data to relay the channel infor-mation, and thus, require multi-block formats to transmit the data(for example, the Group Voice Channel Grant - Explicit ChannelForm).

Single Block Messaging Operation

Implicit messaging usually uses Single Block Messaging consist-ing of single TSBK (trunking signaling block) to provide informationto the SU. It is important to realize that a network configured forexplicit messaging can also use single block messages typicallyused in the overhead, or control messages that are used outsideof the actual channel grant messages that control the channel/fre-quency allocations to the radio (for example, the IDEN_UPMessage).

Besides a single TSBK configuration, it is also important to realizeSingle Block messages can be combined into two or threesequential blocks following the FS (Frame Synch) and NID(Network Identifier). This allows multiple single trunking signalingblocks to be sent consecutively. See TIA/EIA-102.AABB section5 for more details. Do not confuse double or triple TSBK mes-sages as explicit or extended multi-block messages.

2 See the application note "Understanding 800 MHz and VHF/UHF Implicit P25Control Channel Functions using the 2975" for more information on specific message types.

Figure 1 shows the configuration of a single TSBK. As you cansee, the first two octets are used for the LB, P and Opcode infor-mation. The following 8 octets are used for the arguments or theactual message information. The final two octets are used for theTSBK CRC that is a validation check that the data was processedcorrectly.

Figure 1: Single Block TSBK Format

Multi-Block Messaging Operation

Multiple Block Messaging consists of a number of Data Blocks toprovide more robust data information from the RFSS to the SUand vice versa. Again, it is important to realize that a network con-figured for explicit messaging can also use single block messagesas they relate to various overhead, or control messages, besidesthe actual channel grant messages that control the channel/frequency allocations to the radio.

The primary difference between single block messages and multi-block messages is that the single block message formats contain"complete" or whole messages, where the multi-block messagesdisperse the information overone, two or three data blocks. Multi-block messages include a header block that contains informationon how many data blocks are required for the message. Theheader block dictates how many data blocks follow the header.Figure 2 shows the configuration for Multi-Block messages.

Figure 2: Multi-Block Message Formats.

It is important to understand that there are different types of head-er blocks depending on the information being sent. Data (nonvoice) message transmission uses a confirmed header block(requires an acknowledgement that the data has been receivedcorrectly). Voice communication trunking messages use an

unconfirmed header block. For the purpose of this applicationnote we will focus on those used with the various trunked voicemodes, specifically the Group Voice Channel Grant ExplicitChannel Form, the Unit to Unit Voice Channel Grant - ExtendedFormat and the Telephone Interconnect Channel Grant - ExplicitChannel Form.

Figure 3 shows the Alternative Trunking Control Packet HeaderBlock format. This format is used with the above referenced channel grant messages.

Figure 3: Alternative Trunking Control Packet Header Block

The headerblock indicates the type of message being sent via theOpcode field (Alternative Trunking Control Packet Header Block).This is important in the explicit messaging mode. When used withthe channel grant message, the opcode indicates that the following data blocks dictate the TX and RX frequencies that areassociates with the Channel Grant message. For example, theopcode for a Group Voice Channel Grant is %000000. Theopcode is the same for either a single block implicit or a multi-block explicit message. Additional fields are identified asfollows:

BBiitt 77 ooff oocctteett 00:: indicates that the message has multiple blocks.

TThhee AANN bbiitt::TThhiiss bbiitt,, wwiicchh iiss number 6 of octet 0, is set to 0 toindicate that it is unconfirmed.

TThhee IIOO bbiitt:: is set to 1 for outbound or 0 for inbound messages.

FFoorrmmaatt:: is the type of header, either unconfirmed for MultipleBlock Trunking or Alternative Multiple Block Trunking format fortrunking control.

=%10101 - Unconfirmed multi-block trunking control

=%10111 - Alternate multi-block trunking control

TThhee SSAAPP iiddeennttiiffiieerr:: indicates the service access point to where thedata is directed. 61 indicates a non-protected Trunking control,while 63 indicates a protected trunking control message.

TThhee MMFFIIDD:: or the Manufacturer ID identifies the manufacturer IDfor non-standard control channel messaging. Throughout thisdocument, the Manufacturer's ID field is to assume the standardProject 25 Manufacturer's ID of 00.

BBlloocckkss ttoo ffoollllooww:: are a simple indication of the number of blocksafter the header.

HHeeaaddeerr CCRRCC:: is a Cyclical Redundancy Check that verifies thereassembled transmitted bits are coded properly. The user can-


not set this field.

Octet 8 and 9 are defined by the type of message being sent.This is covered later in this application note.

Typical Explicit Control Channel Configuration

Since the P25 standard states that a control channel requires aminimum of the RFSS_STS_BCST, NET_STS_BCST and theIDEN_UP messages, the minimum configuration one wouldexpect to see on an Explicit P25 control channel would includethese messages. Although a "basic" simplified version using onlythe RFSS_STS_BCST and NET_STS_BCST messages would actual-ly work in an implicit messaging environment, it would not workon in Explicit messaging system . That is because the IDEN_UPmessage contains data that is essential to the Explicit radio's oper-ation. The IDEN_UP_VU message is a modified form of the message and is used for the VHF and UHF band operation.

A typical "basic" control channel configuration for Explicit mes-saging is shown in figure 4 where "1" would designate anIDEN_UP message, "2" would designate an RFSS_STS_BCST mes-sage and "3" would designate a NET_STS_BCST message.Please note that the RFSS_STS_BCST and NET_STS_BCST can beeither multi-block format (explicit) or abbreviated form.

Figure 4: An example of a "Basic" P25 Control ChannelConfiguration

Now that we know how the basic messages are put together, weneed to look at the messages closer to gain a better understand-ing of how the control channel operation works in the explicitmode.

Network Status Broadcast Message

The Network Status Broadcast message for multi-block operationcontains fields or information elements as outlined in figure 5.

Figure 5: The Multi-Block Extended Network Status BroadcastMessage

The fields within this message are defined as follows:

LLRRAA:: The LRA defines the region of a registration area in which asubscriber unit may roam without the need to indicate a locationupdate to the network. The registration area may consist of anumber of LRAs. The LRA may be a single site or a collection ofsites of an RFSS. The exact meaning is up to the system design-ers. This is a single octet in length with valid entries of $0000 -$FFFF. See TIA/EIA-102 AABC and AABD.

SSyysstteemm IIDD:: The System ID identifies the home system in a 12-bitfield. The radio may be programmed to operate in up to 8 othersystems through the combination of the (WACN ID + System ID).Valid entries are $001 - $FFE.

BBlloocckkss ttoo FFoollllooww:: In this instance, %0000001.

OOppccooddee:: %111011

TThhee WWAACCNN IIDD:: This field is a 20 bit field that sets the home net-work identity which is hard coded into the radio SU through theuse of a data interface. This dictates the Home network uponwhich the radio can work. Valid entries are $0 0001 - $F FFFE.

CChhaannnneell ((TT)):: This is the actual channel assigned to the Transmitfrequency of the RFSS control channel. The actual calculationused to obtain this will be discussed further on in this applicationnote

CChhaannnneell ((RR)):: This is the actual channel assigned to the receive frequency of the RFSS control channel. The actual calculationused to obtain this will be discussed further on in this applicationnote

SSyysstteemm SSeerrvviiccee CCllaassss:: This is the 8-bit System Service Class fieldthat indicates the basic functions of what the control channel willsupport. The defined values are:

$01 - composite control channel

$02 - no service requests; update control channel only

$04 - backup control channel only

$08 - reserved for future definition

$10 - data service requests only

$20 - voice service requests only

$40 - registration services only

$80 - authentication service only

These values may be ORed together to give different service classdefinitions. A few of the many possibilities are given below forexamples. Other values not listed here are also allowed.

$00 - no services, either trunked or conventional

$F0 - all service, not a backup control channel

RFSS Status Broadcast Message

The RFSS Status Broadcast (RFSS_STS_BCST) messages containfields or information elements as outlined in figure 6.

Figure 6: The Multi-Block Extended RFSS Status BroadcastMessage

The RFSS Status Broadcast message provides additional informa-tion about the sub-system capabilities. It shares some of thesame fields as the Network Status Broadcast Message, but alsohas additional fields as follows:

AA:: This field in the RFSS_STS_BCST message will specify if the sitehas an active network connection with the RFSS controller, i.e.,communication with other sites is possible. A value of 1 will indi-cate a valid RFSS network connection is active.

RRFFSSSS IIDD: The 8-bit RFSS ID field identifies the RF subsystem in aP25 network. Valid entries are $01 - $FE. See TIA/EIA-102 AABDfor more information.

SSiittee IIDD: The 8-bit Site ID field indicates the identity of the site. TheSite ID is unique within an RFSS. Valid entries are $01 - $FE. SeeTIA/EIA-102 AABD for more information.

IDEN_UP and IDEN_UP_VU Messages

IIDDEENN__UUPP MMeessssaaggeess

The IDEN_UP and IDEN_UP_VU messages contain fields or infor-mation elements as outlined in the following figures 7 and 8. Thepurpose of the IDEN_UP messages is to provide a baseline for theradio and then to start calculation of the appropriate TX and RXfrequencies.

The primary difference between the two messages is that theIDEN_UP is used for frequencies outside the VHF/UHF band,where the IDEN_UP_VU is used specifically in the VHF/UHF band(136 MHz to 172 MHz and 380 MHz to 512 MHz). Figure 7 showsthe IDEN_UP message structure.

Figure 7: The IDEN_UP message

For the IDEN_UP message, the fields are defined as follows:

IIddeennttiiffiieerr FFiieelldd:: This is the assigned identifier for the channel. It isa 4-bit field used as a reference for a set of frequencies andchannel characteristics, and it is also used to determine the cor-responding frequency information. Each control channel cansend multiple IDEN_UP messages with different correspondingfrequency information, and there can be up to 16 unique channelset references allowed for each control channel.

BBWW:: This defines the receiver bandwidth and uses this informationin a calculation to determine the actual bandwidth. The formulaused to determine the bandwidth (kHz) is (BW) x (0.125 kHz). TheBW value of zero is reserved and currently not valid.

TTrraannssmmiitt OOffffsseett:: This field represents the separation from the subscriber receive frequency to the subscriber transmit frequency and is a multi-part field where particular bits indicatedifferent functions. The most significant bit, b8 (octet 3, bit 2), isused to describe the relationship between the TX and RXfrequency and is known as the High/Low flag. If the bit is set tozero (0) then the SU transmit frequency is less than the SU receivefrequency. Consequently, if the bit is set to one (1), then the SUtransmit frequency is greater than the SU receive frequency.

The actual value of the offset is then determined by the remain-ing 8 bits. The transmit offset frequency (MHz) is computed as(offset value) x (0.250 MHz).

Therefore, if b8 = 0, then this frequency is subtracted from thecomputed SU RX frequency. If b8 = 1, then this frequency isadded to the computer SU RX frequency.

The value of $00 is reserved to indicate that transmit and receiveoccur on the same frequency. A value of $80 is reserved to indi-cate that there is no standard transmit offset associated.

CChhaannnneell SSppaacciinngg:: This is a frequency multiplier for the channelnumber. It is used as a multiplier in other messages that specifya channel field value. The channel spacing (kHz) is computed as(Channel Spacing) x (0.125 kHz).

BBaassee FFrreeqquueennccyy:: This field determines the absolute frequencywhich is used to determine the frequency for a channel assign-ment as referenced by an Identifier. The frequency is calculatedas (Base Frequency) x (0.000005 MHz). A Base Frequency valueof zero is reserved and not valid.


IDEN_UP_VU Messages

The Identifier Update forVHF/UHF bands is slightly different thanthe IDEN_UP message used outside the VHF and UHF bands. Thetwo fields affected are the BW and Transmit Offset fields. Ofcourse, the base frequency field must contain information regard-ing frequencies that are only valid in the VHF/UHF bands. TheIDEN_UP_VU message uses a shorter (more defined) BW fieldand a longer, more flexible Transmit Offset field. Figure 8 showsthe IDEN_UP_VU message with the longer Transmit Offset VUfield.

Figure 8: The IDEN_UP_VU Message

In the IDEN_UP_VU Message, the two fields affected are definedas follows:

BBWW VVUU: This field (octet 2, bits 3-0) takes on the following valuesfor receiver bandwidth.

%0000 - %0011 reserved

%0100 6.25 kHz receiver bandwidth

%0101 12.5 kHz receiver bandwidth

%0111 - %1111 reserved

These values are essentially non-variable and only the valuesabove are used in a VHF/UHF mode of operation.

The BW field differs in that the IDEN_UP uses a calculated field,whereas the IDEN_UP_VU message uses a shorter, more definedstructure limited to 6.25 kHz and 12.5 kHz, respectively.

TTrraannssmmiitt OOffffsseett VVUU:: This field of 14 bits represents the separationfrom the subscriber receive (SU RX) frequency to the subscribertransmit frequency and is a multi-part field.

The most significant bit (octet 3, bit 7) is used to describe the rela-tionship between the TX and RX frequency. If the bit is set to zero(0) then the SU transmit frequency IS less than the SU receive fre-quency and therefore the sign is -1. Consequently, if the bit is setto one (1), then the SU transmit frequency IS greater than the SUreceive frequency and therefore the sign is +1. This is then fol-lowed by the actual offset information in the remaining 13 bits(octet 3, bits 6-0) and (octet 4 bits 7-2).

It is important to note that the IDEN_UP_VU message can beused with either implicit or explicit messaging.

The IDEN_UP message is used to inform the subscriber unit of thechannel parameters to associate with a specific channel identifi-er. For example, an identifier will have an associated base fre-quency, transmit offset, bandwidth and channel spacing. Eachsystem can have multiple identifiers (up to 16). For implicit P25radios, the IDEN_UP message provides information about the net-work, but in actuality already provides information programmedinto the radio.

Registration in a P25 Trunked System

As with most trunking systems, the P25 format requires that theSU register with the RFSS or network. This is done when the userturns on the mobile radio or when the user moves into a newzone. The primary purpose for registering a SU with the networkis to ensure that only authorized users access the network, andthat the network can track where the SU is located. This reducesthe amount of time and resources that the network needs tolocate the mobile, reducing call setup time and control channelloading.

There are two types of registration in a P25 trunked network, a fullregistration and a location registration. In a full registration thenetwork will check the validity of the SU. A full registration occurswhen the SU is first switched on, enters a new registration area,the user selects a new network or when the RFSS requests registration.

During a location registration, the SU monitors the control chan-nel and then performs a location register in the event the user hasmoved to another site within the coverage area.

In both cases, the registration can be protected (encrypted) forenhanced security. Figure 9 shows a typical full registrationprocess. The blue messages are ISPs and the red messages areOSPs.

Figure 9: Full Registration Process

Unit Registration Request and Group AffiliationRequest (SU Originated ISP)

If we look at the registration process, the basic implementationrequires a registration message and an affiliation message. Thefirst is the Unit Registration Request (U_REG_REQ). This is fromthe mobile to the RFSS and uses a single TSBK format. Thedetailed Unit Registration Request message from the radio isshown in figure 10.

Figure 10: Unit Registration Request (Originated from theMobile Radio)

All the definitions noted earlier for the network status broadcastand RFSS status broadcast messages OSPs apply. In addition,however, there is a new field, not previously defined, calledSource ID.

SSoouurrccee IIDD:: A 24-bit unit identity portion of the unique sub-scriber unit identity. This field along with the WACN ID andSystem ID uniquely addresses a subscriber unit. This can alsobe called the Unit ID.

Note that the subscriber unit has the appropriate WACN ID that isechoed back to the network for verification that the SU acceptedthe Wide Area Communication Network code. This message uti-lizes the Last Block flag identifier field, the encryption P-Bit and theappropriate Opcode that identifies this message as a UnitRegistration Request. The System ID is preprogrammed into theSU as is the Source ID (UID). Manufacturer ID is also shown,which for a P25 system is 00.

Unit Registration Response (RFSS Originated OSP)

This message tells the subscriber unit how its request was han-dled by the network and, if accepted, it includes an assignedSource Address which "tells" the SU what its "new" identifier is.This Source Address is also known as the unit's new "WUID" orworking unit ID. A single TSBK format is used if the WACNID of theRFSS matches the WACN ID in the SU. Figure 11 shows the UnitRegistration Response message.

Figure 11: The Single TSBK Unit Registration Response message.

The Unit Registration Response assigns the new Source Address(WUID) and the RV field. Their functions are defined as follows:

RRVV:: The RV field is a 2-bit field that designates the RegistrationValue. The following values are defined.

%00 = REG_ACCEPT indicates that registration isaccepted.

%01 = REG_FAIL indicates that the RFSS was unable toverify registration.

%10 = REG_DENY indicates that registration is notallowed at the location.

%11 = REG_REFUSED indicates that the WUID is invalidbut the SU need not enter the control channel hunt andthe SU may attempt to re-register after a user stimulus.

See TIA/EIA-102-AABC for more information.

WWUUIIDD:: Within a Registration Area, each Subscriber Unit isassigned a unique abbreviated address known as the WorkingUnit ID (WUID) as indicated by the Source Address field. Once aSU has been assigned a WUID for a Registration Area, the WUIDwill normally be used to address the SU. The SU will use a WUIDto identify target addresses whenever possible or appropriate.The WUID is a 24-bit field.

While within the registration area, a SU will be able to respond tomessages that are addressed to its WUID. Also, while in thedomain of the registration area, a SU shall initiate messages usingits WUID for that registration area. Valid entries are $00 0001 -$FF FFFD. See TIA/EIA-102 AABD for more information.

In some instances the assigned WUID will be the same Unit ID orSource ID received by the SU in the Unit Registration Request.This is an automatic copy back function that simply reassigns theSU's Unit ID as its WUID.

If the WACN ID of the RFSS and the SU do not match, the RFSSsends a multi-block extended message that can be used in con-


junction with the Roaming Address Command(ROAM_ADDR_CMD) from the RFSS to establish a new WACNinto the SU's Roaming Address Stack. This would be contingentupon the network establishing the validity of the SU to allow roam-ing. For this application note, we will assume a valid WACN ID isfound. See TIA/EIA-102.AABC and AABD for more details.

Group Affiliation Request (SU Originated ISP)

After a registration request, the SU will send a Group AffiliationRequest (GRP_AFF_REQ). The detailed Group Affiliation Requestmessage is shown in figure 12

Figure 12: Group Affiliation Request (Originated from theMobile Radio)

The message contains some of the fields that the UnitRegistration Request contains, except that the WACN ID isremoved and the Group ID field is added. This message can beinvoked by the RFSS at any time by broadcasting the Group IDQuery command. The Group ID is pre-programmed into the SU.

GGrroouupp IIDD:: This defines the 16-bit group identifier that togetherwith the WACN ID and System ID uniquely defines a group.

Again, if the WACN ID of the RFSS and the SU do not match, theSU sends a multi-block extended message that includes the SU'sWACN ID. For this application note, we will assume a valid WACNID is found. See TIA/EIA-102.AABC and AABD for more details.

Group Affiliation Response (RFSS Originated OSP)

The Group Affiliation Response message from the networkassigns A Group Address to the SU, i.e., the "WGID". The WGIDworks similarly to the WUID. Figure 13 shows the Group AffiliationResponse message.

Figure 13: The Group Affiliation Response Message

AAnnnnoouunncceemmeenntt GGrroouupp AAddddrreessss:: This field denotes a specialgroup address which addresses a grouping of groups. This isunique within a system. There needs to be some prior arrange-ment to associate the groups to an announcement groupaddress.

WWGGIIDD:: Here the group address is now the WG ID. Within a sys-tem, a SGID or Subscriber Group ID (comprised of the WACN ID,System ID and Group ID) is assigned a unique abbreviatedaddress known as the Working Group ID (WGID). This is a 16-bitfield. See TIA/EIA-102 AABD for more details.

A WGID may be assigned to more than one SU, but will only beassociated with a single SGID at any given time. The 16-bitaddress space of the WGID is customer defined to accommodatedynamic address capability. Valid entries are $0001 -$FFFE. SeeTIA/EIA-102 AABD.

As with the WUID, in some instances the assigned WGID will bethe same subscriber Group ID received by the SU in the Groupaffiliation request. This is an automatic copy back function thatsimply reassigns the SU's Group ID as the WGID.

LLGG:: The LG bit indicates Local or Global affiliation.

LG=0 for Local and

LG=1 for Global

GGAAVV:: The GAV argument is a Group Affiliation Value.

%00 = AFF_ACCEPT indicates affiliation acceptance.

%01 = AFF_FAIL indicates affiliation failure.

%10 = AFF_DENY indicates affiliation denial.

%11 = AFF_REFUSED indicates that the WGID is invalidand that the SU need not enter the control channel huntand the SU may attempt to re-affiliate after a user stimu-lus.

The abbreviated format is used if both the source and target SUhave the same HOME system designation, and the target unitresides in the HOME system. If not, an extended Group AffiliationResponse is sent. For the purposes of this application note, wewill assume that the SU have the same HOME system designationand that the target unit resides in the HOME system.

Understanding Group, Unit and PSTN Call Sequences

As the call is initiated and managed, additional messages areinvolved in the call sequence. For example, Figure 14 shows therequired messages to establish a P25 unit to group voice call afterthe Unit Registration and Group Affiliation process is complete.

Figure 14: Unit to Group Call Sequence Chart

Once the mobile is registered and has affiliated, the only com-mands required to move the mobile to a particular channel, is toissue a channel grant after a Group Voice Request from an SU.

The Unit to Unit channel grant is more detailed in that therequired communications from the Source SU through the RFSSto the Destination SU requires the issuing of an answer requestfrom the RFSS and an answer response from the Destination SU.Figure 15 shows the call sequence for a Unit to Unit call.

Figure 15: Unit to Unit Call Sequence Chart

The Unit to PSTN call requires interaction between the RFSS andthe PSTN. The P25 Air interface messages require a PSTN inter-connect request, either using a dial request or a PSTN request.The dial request includes actual digits dialed, and can be up to 10digits - or more than 10 digits, using the extended or multi-blockmessaging. The PSTN request is a user defined reference

associated with a dialing sequence. Figure 16 shows the Unit toPSTN interconnect call process.

Figure 16: Unit to PSTN Call Sequence Chart

Establishing a Group Call in an Explicit P25 TrunkedSystem

As in any communication system, a user initiates a call. This callcan be defined as two types, either a SU originated call or a SUterminated call. The difference is that the SU either generatesthe call request or the SU receives a call request. Either actionends up with a channel grant being delivered to the SU to beginvoice communications. In Figure 14 above, the group callsequence begins with a Group Voice Request from the SU to theRFSS. Let's look further at the messages involved in this transaction.

Group Voice Request (SU Originated ISP)

The SU will send this when initiating the Unit to Group callsequence. Here, an additional message is sent, the Group VoiceRequest (GRP_V_REQ). This message is shown in Figure 17.

Figure 17: Group Voice Request Message

Note that this message reflects the newly created WG ID as theGroup Address. It also contains a new field as follows:

SSeerrvviiccee OOppttiioonnss:: Service requests and service grants allow forspecial service extensions called the Service Options field. Thisfeature provides extended flexibility to tailor the requested serv-ice to the needs of the requesting unit or process capability of thesystem. Figure 18 shows the Service Option field definition.


Figure 18: The Service Option Octet

The following are definitions for each bit:

7: EEmmeerrggeennccyy - this is the status indication to determine ifthis service is to be specially processed as an emergency service.

0 = Non-emergency - indicates the normal processing status.

1 = Emergency - indicates special processingrequired.

6: PPrrootteecctteedd - indicates whether the resources (other thancontrol channel resources) to be associated with thisservice should be presented in protected mode (e.g.encrypted) or not.

0 = not protected - indicates normal mode pres-entation for the resource(s).

1 = protected - indicates protection mode presen-tation for the resource(s).

5: DDuupplleexx - indicates the way the channel resource is to beutilized by the unit(s) involved in the call.

0 = Half duplex - indicates the unit will be capableof transmitting but not simultaneouslyreceiving on an assigned channel.

1 = Full duplex - indicates the unit will be capableof simultaneous transmit and receive on anassigned channel.

4: MMooddee - this is the indication of whether this service session should be accomplished in a packet mode orcircuit mode.

0 = Circuit mode - will utilize resources capable ofsupporting circuit operation.

1 = Packet mode - will utilize resources capable ofsupporting packet operation.

3: RReesseerrvveedd - currently set to null (0)

2-0: PPrriioorriittyy lleevveell - indicates the relative importance attrib-uted to this service where:

%111 Highest (top)

%110 System definable


%100 Default



%001 Lowest (bottom)

%000 Reserved

Group Voice Channel Grant (RFSS Originated OSP)

It is important to note that the Group Voice Channel Grant con-tains both the "WUID" and "WGID". The fields are derived fromthe previous dialog between the SU and the RFSS during the reg-istration and affiliation processes, and ensure that the SU onlyresponds to channel grants that apply to his unique unit andgroup identifiers.

After the Group Voice Request is received, the RFSS respondswith a Group Voice Channel Grant message(GRP_V_CH_GRANT). In the explicit mode of operation, thismessage is sent as a multi-block message. Figure 19 shows theGroup Voice Channel Grant Explicit message. Notice the channelinformation section shown in Octets 2, 3, 4 and 5 of the last datablock where the actual TX and RX frequencies are set.

Figure 19: The Group Voice Channel Grant Explicit Message

The Channel (T) field identifies the transmit frequency of the FNE(the SU RX) and the Channel (R) field identifies the receive fre-quency for the FNE (the SU TX). Each Channel field is actuallycomprised of two elements of information. The first element is theChannel Identifier, which determines the band of frequencies thatare associated with the Channel Number. As we discussed earli-er, there are up to 16 unique identifiers per site and each identi-fierwill include information such as base frequency, channel spac-ing, transmit offset and bandwidth. In the explicit mode of oper-ation, this information coincides with the information sent in theIDEN_UP_VU message.

The second element is the Channel Number, which contains avalue to be used to derive the actual frequency of the channel.The SU receive frequency is computed from the base frequencyand channel spacing information from the IDEN_UP message andchannel number from the GRP_V_CH_GRANT as follows:

SU RX = (Base Frequency) + (Channel Number) x (ChannelSpacing). Figure 20 shows the channel information broken intoits two elements.

Figure 20: The Channel Information

How Mobiles Calculate the RX and TX Frequencies

In the explicit mode of operation, the SU will use the channelidentifier information and the channel number in conjunction withthe IDEN_UP_VU message to calculate the SU RX and SU TXfrequencies.

Calculating the SU RX Frequency

The SU RX Frequency is calculated as follows:

SU RX = (Base Frequency) + (Channel Number) x (ChannelSpacing).

NNoottee:: Base Frequency is calculated as (Base Frequency) x(0.000005 MHz) from the IDEN_UP_VU message. See TIA/EIA-102 AABC-2 section 2.3.5.

We know that both the Base Frequency and the Channel Spacingare derived from the IDEN_UP_VU message. The transmittedChannel Number from the Group Voice Channel Grant Explicitmessage becomes the key part of the equation for frequencyassignments.

Calculating the SU TX Frequency

The SU transmit base frequency is computed as follows:

Using the Transmit Offset VU field from the IDEN_UP_VU mes-sage, the SU TX Frequency is simply: Sign (Transmit Offset xChannel Spacing x 12500).

That value is then added to the SU RX frequency.

Establishing a Unit to Unit Call in an Explicit P25Trunked System

The Unit to Unit call process is very similar to the Group callprocess with the exception that the group voice request isreplaced by the unit to unit voice service request (UU_V_REQ).The UU_V_REQ can be either a single block or multi-block format.The single block operates in the same manner as the GRP_V_REQwhereas the multi-block is an optional configuration that adds theWACN ID and the System ID to provide a more flexible addressfor the target SU if it is outside its home system. For this appli-cation note, we will use the single block format. See TIA/EIA-102.AABC for more details. Figure 21 shows the UU_V_REQ sin-gle block format.

Figure 21: The UU_V_REQ Message Single Block Format

As can be seen, everything is the same as the GRP_V_REQ mes-sage except that the Target ID has replaced the Group Address.Once the UU_V_REQ has been received, the RFSS sends aUU_ANS_REQ which the target SU picks up and, if available,responds to the RFSS with a UU_ANS_RSP message indicatingthat it will accept the call. Figure 22 shows the UU_ANS_REQmessage.

Figure 22: The UU_ANS_REQ Message

In this outbound message (OSP), the Target Address replaces theTarget ID and the Source ID replaces the Source Address com-pared to the UU_V_REQ which is an inbound message (ISP).Again, the multi-block mode can be invoked if either user is out-side the home system. Once this message has been issued, theTargeted SU for the unit to unit call can reply with theUU_ANS_RSP which indicates that the user is available to acceptthe call. Figure 23 shows the UU_ANS_RSP.

Figure 23: The UU_ANS_RSP message


With this message response, the user can send one of three"Answer Responses". These include:

$20 - Proceed

$21 - Deny

$22 - Wait

With an acceptable response (Proceed), the RFSS then sends aUU_V_CH_GRNT message assigning the two units in the call to aparticular frequency. In this instance, we will show the explicitmode (multi-block) messaging in figure 24. After the call is done,the SU can issue a cancel service request to terminate the call.

Figure 24: The UU_V_CH_GRANT Message.

The explicit mode of operation is shown with both the transmitand receiver parameters detailed along with the extendedaddressing including the WACN ID and source System ID.

Establishing a PSTN Call in an Explicit P25 TrunkedSystem

Similarly to the way the Group call and Unit to Unit call operates,the PSTN differs in that the call can originate from the SU to theRFSS and then on to the PSTN, or it can originate from the PSTNto the SU through the RFSS. If the SU originates the request, itcan originate two types of calls. One type is a PSTN requestknown as a TELE_INT_PSTN_REQ where the SU sends a PSTNaddress that is 8 bits refers to a PSTN dialing sequence refer-enced in the network. The other type is a TELE_INT_DIAL_REQthat specifically (explicit) indicates the numbers to be dialed.Figure 25 shows the TELE_INT_DIAL_REQ format and Figure 26shows the TELE_INT_PSTN_REQ format.

Figure 25: The TELE_INT_DIAL_REQ Message

This particular message uses a different format from the previousmessages in that the OPCODE field in the header block has beenreplaced by a Pad Octet Count. This is actually the standardTrunking Control Packet Header Block and it is used becausethere is another alternative that allows up to 34 digits to be dialedand requires another data block to accomplish this. The pad octetcount allows the RFSS to determine the number of blocks to fol-low. See TIA/EIA-102.AABB for more details. As we can see, theactual dialed digits are included in this message.

Figure 26: The PSTN Request Message

Once the dial or PSTN request has been received, the RFSS negotiates with the PSTN link to establish the call. Once the RFSSreceives a response from the PSTN to continue, it issues a channel grant in the form of a TELE_INT_CH_GRANT message asshow in Figure 27. Once the call is done, the SU can issue a cancel service request to terminate the call.

Figure 27: The TELE_INT_CH_GRANT Message

Testing P25 Trunking Radios in an Explicit mode

Since P25 Trunking systems utilize digital voice channels con-trolled by a digital control channel, we do not typically need to testthe inbound control channel (ISP) for parametric performance.However, we do need to allow for setup and testing of the inter-action between the control channel and the transition to the traf-fic channel. These tests are called protocol tests, in which we testthe radio's ability to respond to commands from the radio test set.

In order to do this, the test system needs to be able to emulatesome of the protocol (OSPs) that the repeater sends out. In addi-tion, the system needs to be able to respond to the ISPs themobile is sending and react appropriately to get the unit onto atraffic channel for proper parametric testing. While on the trafficchannel, the test system also needs to keep in communicationwith the mobile with control messages that are "associated" orembedded with the voice call information. This is done throughP25 specified LDU (logical data units) messages. These data unitscarry both voice and data information during a "call".

Being RF based, these systems also require the ability to test andverify parametric performance including frequency error, power,modulation accuracy and other parameters.

Using the 2975 to perform advanced P25 trunkingtests

The 2975 provides powerful test features for P25 Trunked systems. These tests include the following:

1. The ability to emulate a P25 repeater station and initiate a system originated call. Either implicit or explicit. With the 2975,you are not locked into a specific test sequence for the radio.You decide what pass and fail means.

2. The ability to handle a P25 mobile initiated call. Either implic-it or explicit.

3. The ability to perform full digital test functions.

The 2975 provides access to message elements and fields thatother P25 testers do not allow. This gives the operator added flex-ibility in testing functionality, especially interoperability testingbetween mobiles and P25 networks.

P25 Trunking Control and Traffic Channel Setup

Setting up the control and traffic channel is the first thing thatneeds to be accomplished to begin testing P25 Trunked radios.This application note will show you how to enable the P25Repeater Simulator for testing mobiles. In all test modes for P25mobile systems, the instrument should be in the duplex mode. Toenable the duplex mode, click on the screen selection buttonnext to the time at the very upper left of the screen or select, as shown in Figure 28.

Figure 28: Selecting Duplex Mode

From there, we want to enable the Repeater Simulator. To do this,select the "Options" button next to the Duplex display at the topright of the display or select , and then select"P25 Repeater Sim". See Figure 29 for this selection. Note thatwe have also selected the Spectrum Analyzer for display.

Figure 29: Selecting the P25 Repeater Simulator

To enable a valid control channel, we need to set up the propercontrol and traffic channel numbers. This is fairly straightforwardfor 800 MHz operation, in that the channels are defined with a setfrequency assignment. The duplex offset is also standard at 45 MHz. We simply enter the proper channel number or fre-quency in the CC and VC data fields. See figure 30. Refer to theapplication note "Understanding 800 MHz and VHF/UHF ImplicitP25 Control Channel Functions using the 2975" for more details.


Figure 30: Setting-up the Control Channel and Voice (Traffic)Channel - 800 MHz band

ForVHF/UHF implicit operation, things become a little more com-plex. You will note that upon selecting the U/V mode, an addi-tional button named "CONFIG" pops up under the REG/AFF indi-cator section. See Figure 31 for more detail. TheVHF/UHF/700MHz Other Band P25 Trunking Option (OPT 14)needs to be installed before being able to set up this feature.

Figure 31: The VHF/UHF P25 Implicit Trunking Mode

Expanding the "CONFIG" button reveals in detail the ChannelIdentifier information we studied earlier in this application note.Figure 32 shows the configuration screen.

Figure 32: The Configuration Screen for VHF/UHF mode ofoperation

As you can see, the configuration screen shows all the parame-ters associated with the IDEN_UP_VU messages. The BaseFrequency, bandwidth, transmit offset and channel spacing areset equivalent to the IDEN_UP_VU information or the radio's inter-nal programming. This is the same for either implicit or explicitset-up. The userneeds to be very familiarwith this feature in orderfor this table to be set up correctly. Note that the 2975 sends oneIDEN_UP_VU message at any given time, but allows you to selectmultiple configurations for this message.

Setting Explicit Control Channel Protocol Elements

Now that we understand the control channel broadcast mes-sages, the registration and group affiliation process and the vari-ous call sequences, we can move on with setting up the 2975 toperform not only parametric tests, but also protocol tests. Wehave the control and traffic channel setup, however, we need toset additional parameters before proceeding. For proper opera-tion, we need to enter a valid WACN ID and System ID, whichwe've already reviewed. We also need to set the NAC.

The NAC is the Network Access Code that is an 8-bit field. WithP25 trunking systems, the NAC is typically the same as the P25System ID, although they may be configured differently to provideadded flexibility for the network operator. Figure 33 shows howto set up the WACN ID, NAC, System ID and the MFID -Manufacturers ID (typically "00" for P25 compliant messages).Setting the WACN ID, NAC and System ID along with configuringthe IDEN_UP_VU message, is the minimum configuration fortesting the mobile unit.

Figure 33: Setting the WACN ID, NAC, System ID and MFID

While the difference between the implicit and explicit mode ofoperation is extensive, the 2975 makes that difference almosttransparent to the user. With Option 14, the VHF or UHF P25trunked radio feature is enabled on the 2975 for the implicit modeof operation. By enabling Options 22 and/or 23, this feature nowexpands the 2975's VHF and UHF operation to include emulationof the multiple block message formats discussed earlier. Option22 will enable the Group voice channel grant message, whileoption 23 will enable more extended call support, including theUnit to Unit and PSTN interconnect call capabilities. With theexplicit mode of operation, the 2975 is sending an IDEN_UP_VUmessage as seen in Figure 4.

The main differences between implicit and explicit mode can bestbe seen when the "downlink data" button is selected in theVHF/UHF mode. See Figure 34 to see how to select the "down-link data" functions.

Figure 34: Selecting the Downlink Data Functions

Once selected, the user can select a variety of parameters asso-ciated with the OSP set up of the 2975. This includes the abilityto control the various parameters included with the RFSS StatusBroadcast and Network Status Broadcast messages as well as theGroup Voice Channel Grant as was covered earlier in this appli-cation note. Figure 35 shows how to configure the 2975 foremu-lation of these parameters.

Figure 35 Setting the Various Downlink Parameters for ControlChannel Emulation.

Notice that when the Explicit Mode (OPT22) of operation isenabled, the user is given the option to toggle between an implic-it and explicit mode of operation. After selecting the ExplicitMode of operation and closing the downlink data box, we canobserve the difference between the Implicit and Explicit mode ofoperation. Compare Figure 36 to Figure 31. You'll notice that theControl Channel and Voice Channel parameters now have theexplicit TX and RX frequencies associated with the Explicit modeof operation. These TX and RX frequencies are based on the con-figuration data entered earlier (IDEN_UP_VU data). In addition, anew field appears to the left of the channel number and frequen-cy fields. This is the channel identifier used to reference whatchannel combination was defined earlier.

Figure 36: The Explicit Mode Control Channel and VoiceChannel

Setting the specific TX and RX frequencies and enabling theExplicit Mode of operation are the only user operations that arerequired to switch the 2975 from an implicit mode to an explicitmode using multi-block messaging.

As you can see, there is also a feature area designated as theAuto-Copy area, see Figure 35. This area deals with the WUIDand WGID issues discussed earlier. By enabling the auto-copyfunction, the user can have the 2975 echo back (copy) the SU'spre-programmed Unit ID (Registration) and/or the Group ID(Affiliation) by enabling the appropriate box (in red). Enabling theVoice Request means that during the Group Voice Channel Grant,the system will use the Group ID received from the SU instead ofthe WGID in the message parameters.

Disabling any of these functions means that the pre-programmedWGID and WUID would be used as needed. This can be pro-grammed in the top line of the P25 Trunking DownlinkConfiguration screen.

Uplink Data Analysis

The 2975 will automatically decode a number of fields from themobile when the unit registers and affiliates with the 2975. Thisfeature can be very helpful in determining if the unit has beensetup correctly for use on the network. On the primary P25 trunk-ing tile, you will see the two indicators for the GID or Group IDused for initiating group calls, and the UID or Unit ID (Source ID).Only the 2975 provides this capability for testing P25 mobiles.

Now that we understand the fields in each of the messages usedduring a registration and affiliation process, and during a unit togroup voice call, we can use the 2975 to decode these fields toassist in testing interoperability of various SU's and systems.Figure 37 shows the 2975's P25 Trunking Uplink Data Decodescreen.


Figure 37: The P25 Trunking Uplink Configuration DecodeScreen.

These fields are derived from the SU's U_REG_REQ andGRP_AFF_REQ messages sent during the registration and affilia-tion processes discussed earlier. These are not user definablefields.

Testing with the 2975 - Group Voice Channel ExplicitMode

We can now perform either a mobile originated call or a repeateroriginated call. To perform mobile radio transmitter tests, simplykey the mobile unit and the 2975 will automatically assign the unitto the proper voice channel for performing parametric tests usingthe appropriate message format as selected by the implicit orexplicit function.

Figure 38 shows the 2975 setup in the Duplex Mode with the P25repeater simulator running in the VHF band and with a SpectrumAnalyzer tile enabled to view the spectrum from the SU. Noticethat the SU has registered with the 2975 as seen by the lit REGREC field.

Figure 38: P25 Trunking Control Channel Screen with theSpectrum Analyzer Enabled Showing a Unit Registration

Received from the SU

Figure 39 shows the Group Affiliation message received from theSU to the 2975

Figure 39: P25 Trunking Control Channel Screen with theSpectrum Analyzer Enabled Showing a Group Affiliation

Received from the SU

Figure 40 shows the SU originating a call, as seen by the lit PTTREC (push to talk received) field on the P25 Trunking tile.

Figure 40: P25 Trunking Control Channel Screen with theSpectrum Analyzer Enabled Showing a SU Originated Call (PTT

REC)

Note that the duplex receiver and generator frequencies havechanged since the unit is now on the voice channel with the pre-viously defined duplex offset as established in the P25 channelconfiguration table.

To enable a call from the 2975 to the SU, simply de-key the radioand select the PTT OFF button. This will toggle to a PTT ON modeand the 2975 will send a group voice channel grant message andthen transmit the appropriate voice data as selected by the user(1011 tone, Speech, etc…) and keep the call active until the PTTON is toggled to the PTT OFF mode.

With Option 22, the group voice mode is the only explicit modeoperating, however, with Option 23 enabled, the user can alsoselect unit to unit or PSTN interconnect modes.

Testing with the 2975 - Unit to Unit Explicit Mode

If the SU is originating a unit to unit call, the 2975 will automati-cally determine the type of call coming in and respond appropri-ately. For example, if a unit to unit call is received from the SU,the PTT REC field will automatically change to indicate that a unitto unit call request has been received. The 2975 then accept thecall and send an UU_ANS_REQ to the radio and then aUU_V_CH_GRANT. See Figure 41 that shows a Unit to UnitRequest Received.

Figure 41: P25 Trunking Control Channel Screen with theSpectrum Analyzer Enabled Showing a SU Originated Unit to

Unit Call (UU REC)

However, if the user wants to initiate a unit to unit call from the2975 to the SU, then the 2975, when the PTT OFF button is selected (with Option 23 enabled only), will display the ability toinitiate either a unit to unit call or PSTN call. See Figure 42.

Figure 42: With Options 22 and 23 enabled, the 2975 Allowsfor Explicit Group, Unit to Unit and PSTN Calls

With the unit to unit call, the 2975 will send to the SU theUU_ANS_REQ message and then wait for the appropriate userinteraction (that results in the generation of the UU_ANS_RSPmessage) to the 2975 that allows the call to be accepted. The2975 will display the received response from the SU (Proceed,Deny or Wait) and display that on the PTT REC field of the trunk-ing tile. See Figure 43.

Figure 43: The 2975 Displaying the R WAIT State Received fromthe SU during a Unit to Unit Call

If a "Proceed" is received, the 2975 then finishes the communi-cation with a UU_V_CH_GRANT message to complete the callwith the appropriate user selected voice data. See Figure 44 andnote the changed frequencies for the transmitter and receiver.

Figure 44: The 2975 Displaying the R GO (Proceed) StateReceived from the SU during a Unit to Unit Call

In addition to seeing the various call processes on the trunkingtile, the 2975 allows the user to set various parameters being sentto the radio during the Unit to Unit call process. During the Unitto Unit call set up, the Source ID, Source Address and TargetAddress are sent to the target SU and then to both the target andsource SU in the channel grant. The 2975 can individually setthese parameters to emulate a specific user on a network. SeeFigure 45.


Figure 45: Configuring the Source ID, Source Address andTarget Address in the 2975

Testing with the 2975 - PSTN Explicit Mode

If the SU is originating a PSTN call, the 2975 will automaticallydetermine the type of call coming in (either a TEL_INT_DIAL_REQorTEL_INT_PSTN_REQ) and respond appropriately. The PTT RECfield will automatically change to indicate that a PSTN call requesthas been received. The 2975 will then accept the call and senda TEL_INT_CH_GRANT to the radio. See Figure 46.

Figure 46: The 2975 Displaying the Dial Request Received fromthe SU during a PSTN Dial Call

If the user wants to initiate a PSTN call from the 2975 to the SU,then the 2975, when the PTT OFF button is select (with Option 23enabled only), will again display the ability to initiate either a unitto unit call or PSTN call (See Figure 41 above). The 2975 will thensend a channel grant to the radio. See Figure 47.

Figure 47: The 2975 Generating a PSTN Call to the SU

The PSTN mode allows the user to define the number that will besent to the radio. This is important information, as the user canthen test how the radio recovers that number and then display oroptionally correlate that information to a user identifier (i.e. "Bob"calling). See Figure 48 to view the associated telephone numberand timer information.

Figure 48: Setting-up the 2975 to Emulate the Dialing Number.

Parametric AnalysisBesides the advanced protocol test capabilities, the 2975 has theability to perform extensive parametric analysis.

Once we have the control channel set up and running, and themobile is talking with the 2975, we can then select any number ofoptions to run. This includes the Oscilloscope, SpectrumAnalyzer, Meter Panel, Power Meter, RSSI Meter, etc…. To enablethese functions, select the "Options" button or select, and select the combinations you wish.Remember that not all screens can be displayed at one time andyou will be limited to two 1/4 tile "Meter" screens or one 1/2 tileOscilloscope/Spectrum analyzer screen when run in conjunctionwith the P25 Control Channel simulator screen in the duplexmode. More screen modes can be defined in the user mode or the receiver mode .

The 2975 has every instrument constantly running in an opera-tional state, and by simply changing modes, we change the "view"of the instrument. As seen in Figure 49, the 2975 receiver shows the receive frequency, power meter, frequen-cy error meter, modulation accuracy meter, and the traffic uplinkdecode information.

Figure 49: The Receiver (Transmitter Test) mode with Power,Frequency Error and Modulation Meters and the P25 Uplink

Decode Tile

Notice that the meters display green bars. This is because priorto conducting the test, we set pre-determined pass/fail limits foreach of the parameters. We know that the radio is within therange of performance for Power, RF Frequency Error andModulation Error.

To perform mobile receiver measurements, enable the PTT modeby selecting the PTT button directly below the ON/OFF button onthe repeater screen. This will cause a repeater originated callrequest to be sent to the mobile and the mobile will then beassigned to a voice channel in a "listen" (receive) mode.

We can now enable a unique sensitivity test called the "Speech"mode. The speech mode is a special voice file that provides apre-recorded voice pattern that allows the test professional toquickly determine the receiver's sensitivity. This is accomplishedby listening to the pre-recorded audio voice patterns and thenreducing the RF level until the voice starts to break up or soundsslurred. This test is unique to the 2975 and is another way thatAeroflex is improving technician productivity. Figure 50 showshow to select the speech mode and Figure 51 shows where toadjust the RF level. Once you hit a level where the voice is break-ing up, that become the receiver's "realistic" sensitivity. Tests haveshown that this mode of testing typically correlates to a 5% BERtest, commonly used for digital receiver testing, by a few dB.

Figure 50: Setting the 2975 up for P25 Sensitivity Testing Usingthe Speech Mode.

Figure 51: Decrease the RF Output until Speech Quality Degrades

Conclusion

The 2975 provides some of the most advanced testing functionsever presented in a communication test set. For testing P25 sys-tems, the 2975 truly allows for dynamic testing by means of truerepeater functionality. The 2975 provides the test professionalwith the most advanced wireless test set on the market for P25and other PMR systems.

Part No. 46891/942, Issue 1, 08/06

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As we are always seeking to improve our products,the information in this document gives only a generalindication of the product capacity, performance andsuitability, none of which shall form part of any con-tract. We reserve the right to make design changeswithout notice. All trademarks are acknowledged. Parent company Aeroflex, Inc. ©Aeroflex 2006.

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